I'm going to be buying a car soon. I'm really interested in getting a used Geo/Chevy Metro, Suzuki Swift or Pontiac Firefly. I live in the US, so I don't know how likely it is I'll be getting a Firefly.

Do you know what year and class of Metro or its sisters was rated with the best fuel economy?

There was a specific model made to best the 'normal' Metro. It was tagged the Metro XFI. Others here will be able to give you specifics, but it got a significant boost in mileage above the normal Metro.